The invention relates to a soundboard of composite fibre material construction comprising at least one composite fibre laminate consisting of long fibres and carrier material, such soundboard being for use in an acoustic musical instrument, particularly a bowed stringed instrument.
However, the invention can also be used for other acoustic musical instruments (such as guitars and pianos) which are provided with a resonant body or resonant back-plate.
In recent years attempts have also been made to produce the soundboards of acoustic musical instruments in composite fibre material construction. Structures of composite fibre material construction generally consist of elongate fibres which are preferably oriented in certain directions and a carrier or matrix material which is generally a thermosetting or thermoplastic plastics material. In the preferred embodiment of the invention this is an epoxy resin system.
The previous efforts to produce soundboards of composite fibre material construction intended for acoustic musical instruments are aimed without exception at copying as well as possible the acoustic characteristics of the wood which is to be substituted. Examples of these attempts in the previously known prior art are provided for instance by DE 37 38 459 A1, EP 0 433 430 B1, U.S. Pat. No. 5,895,872 and U.S. Pat. No. 5,905,219. Thus DE 37 38 459 A1 aims at xe2x80x9ca macroscopic heterogeneity almost equal to the woodxe2x80x9d and states as the object that xe2x80x9cthe composite materialxe2x80x9d should xe2x80x9chave similar characteristics to sprucexe2x80x9d.
An unsatisfactory feature of these previously known soundboards of composite fibre material construction appears to be that from the acoustic point of view they are equivalent but in no way superior to very good solid wood soundboards of traditional construction.
The object of the invention, therefore, is to create a soundboard of composite fibre material construction which has a perceptibly better acoustic quality by comparison with excellent soundboards of traditional construction. In particular the soundboard according to the invention should have substantially higher radiated power whilst retaining the usual and desirable timbre of a solid wood soundboard.
This object is achieved according to the invention in that the core plate has at least one recess surrounded by material zones of the core plate within the area defined by the outline of the soundboard, the total volume of all recesses amounting at most to 80%, preferably between 20 and 45%, of the total volume of the core plate filled with material.
Composite fibre sandwich structures are basically constructed in such a way that a core plate of low density is provided on both sides with composite fibre laminate layers. In this case the bending strength of the structure is heavily dependent upon the thickness of the core plate. Core plates of composite fibre sandwich constructions are frequently produced from hard foam materials. Balsa wood is used for the preferred embodiment of the invention. The fibre laminate can be produced by means of layered fibre structures, fibre meshes, hand lay-up laminated individual rovings or the like, as prepreg or by means of a suitable manufacturing process. Layered fibre structures in the form of prepregs are preferably used in the construction according to the invention. These are preferably single-layer and at the same time multidirectional.
In detail, the invention is based upon the following considerations and tests:
The vibration levels of the characteristic vibrations are crucial for the sound radiation of the instrument. They are dependent upon the vibrating mass of the soundboard. The vibration resistance (so-called impedance) which the soundboard opposes to the exciting alternating force generated by the string vibrations is greater the higher the vibrating mass of the soundboard is. In order to achieve high vibrating speeds (so-called velocity) of the soundboard and thus the most effective possible sound radiation of the instrument, with a given excitation force the lowest possible vibration resistance and thus the lowest possible vibrating mass are necessary.
For these reasons it is sensible to reduce the vibrating mass of the soundboard of composite fibre material construction.
It might be thought that the required reduction of the vibrating mass could be achieved by reducing the thickness of the core plate. This possibility has proved unfavourable in so far as a reduction in the thickness of the core plate is accompanied by a reduction in the quotient of bending strength and total density. The bending strength should be high in order to achieve large-area in phase antinodes of the characteristic vibrations of the soundboard and to shift downwards the so-called cutoff frequency [Cremer, L., Heckl, M.: xe2x80x9cKxc3x6rperschallxe2x80x9d, Berlin 1996, page 498], below which no effective sound radiation is possible any longer, and to avoid hydrodynamic short circuits [loc. cit. page 477].
A further possibility for reducing the vibrating mass of the soundboard would be to reduce the area or the weight per unit area of the fibre laminate. Here too there is a danger of a reduction in the quotient of bending strength and total density.
A third possibility for reducing the vibrating mass of the soundboard could be seen in the reduction of the board dimensions. However, this would have the disadvantage that the characteristic frequencies would be shifted upwards and as a result the timbres of the instrument would be changed in an undesirable manner.
With these considerations as a starting point, therefore, the invention follows a fundamentally different route in order to reduce the vibrating mass of the soundboard of composite fibre material construction: Recesses are provided in the core plate.
The vibrating mass of the soundboard which is reduced according to the invention enables instruments to be produced with an improved acoustic efficiency relative to the prior art.
Some embodiments of the invention are explained in greater detail below with reference to the drawings.